Last night, Hillary Clinton, the US secretary of state, said the five permanent security council members plus Germany "continues to be unified" in talks on sanctions. "There will be a great deal of further consultation not only among the [six], but other members of the security council and other [UN] member nations during the next weeks," she said.

According to officials with knowledge of the talks, an agreement to begin drafting a security council resolution was reached in a telephone call involving representatives of the US, China, Russia, Britain, France and Germany.

The negotiations will now move to New York, where diplomats will hammer out a sanctions package. Barack Obama said on Tuesday he hoped the UN would pass a sanctions resolution within "weeks".

Bill Burton, a White House spokesman, said yesterday there was a real sense of urgency about applying pressure to Iran. "There are some very intense conversations happening at the United Nations right now that we're able to make some real progress on."

Mark Toner, a US state department spokesman, confirmed the telephone conference had taken place and that the US had been represented by number three at the state department, William Burns.

Three rounds of sanctions have already been imposed on Iran. The US claims Iran is covertly seeking a nuclear weapon capability while Iran's president, Mahmoud Ahmadinejad, pictured below, says its nuclear industry is for peaceful power generation.

Negotiations over sanctions have taken months in the past and although Obama says he would like a deal done this spring, they are likely to be protracted again. China and Russia want a narrower set of measures than the Americans and western Europeans have been seeking.

Following the revelation last September that Tehran had been building a covert uranium enrichment plant near Qom, and the collapse of a compromise deal by which Iran would export the bulk of its enriched uranium stockpile for processing, Russia's president, Dmitry Medvedev, had agreed the need for new sanctions.

But until yesterday, Beijing had held out against US-led pressure to begin drafting a resolution.

The talks were complicated by other flashpoints in US-China relations, particularly American arms sales to Taiwan in January and Obama's meeting with the Dalai Lama in February.

The agreement yesterday came after a similar conference call a week ago in which China participated after weeks of stalling.

Burton said the White House was confident that it will be able to work with China to apply "meaningful" pressure.

Bringing China to the negotiating table is seen in Washington as a diplomatic breakthrough, but the degree of Obama's success in winning the argument will be measured by the final terms of the UN resolution, and on how long it takes to agree.

China is in the process of changing its delegation at the UN, a move that could delay negotiations further. Some officials said they expected the security council talks to drag on until June.

The US had originally sought broad sanctions against Iran's energy sector. Russia and China have said the measures are targeted against individuals and institutions directly linked with Iran's nuclear and missile programme.

The news of diplomatic progress towards sanctions coincided with a report that an Iranian nuclear scientist who went missing in Saudi Arabia last summer had been persuaded to defect by the CIA and had been resettled in the US.

ABC News said the defection of Shahram Amiri was part of a "long-planned" CIA operation.

"The CIA reportedly approached the scientist in Iran through an intermediary who made an offer of resettlement on behalf of the US," the broadcaster said.

It echoed earlier speculation that Amiri, an expert on radioactive isotopes at Malek Ashtar University in Tehran, had been persuaded to defect while on a pilgrimage to Mecca late last May or early June.